This invention relates to a power converter such as an inverter and more particularly to a method and apparatus for controlling a power converter which is preferably used for a winding machine for an elevator.
Inverters have been widely used for controlling elevators using induction motors for driving winding machines. Power converters including such inverters have been mainly controlled by analog control systems. Such analog control systems, however, provide several problems including their output fluctuation and limited setting accuracy; the output fluctuation will be caused by the secular changes of several components used in the systems.
In order to solve these problems, several digital control systems have been proposed.
One example thereof is disclosed in JP-A-57-25171. In the system disclosed therein, pulse width data is previously stored, during the absence of the processing by a microcomputer, in a timer externally attached in the bus of the microcomputer, and the microcomputer, when interrupted by the timer, only supplies an actuation instruction to the timer, thereafter performing a scheduled processing, so that the microcomputer doesn't need to perform the output processing. This system permits the microcomputer to concentrate on the processings of the reference to a data table for acquisition of the pulse width data and the data working so that the performance thereof may be improved.
Another example is disclosed in an article entitled "Magnetic Flux Type Realtime Processing PWM (Pulse Width Modulation) Control by One-chip Microcomputer" in the Proceedings of the Institute of Electrical Engineers of Japan, Vol. 105, No. 6, pp 531-538, (June 1985), 60-B61. This example is a system of performing a PWM control in a direct connection of an input/output port incorporated in a one-chip microcomputer and a pulse amplifier. This system provides the advantages that the circuit construction is simplified since the timer can be incorporated in the one-chip microcomputer and the distribution circuit can be implemented in software, and the abnormality of the port can be covered in some degree by the self-diagnosis function of the microcomputer.
However, the former system involves the following problems since a timer, distribution circuit, etc. must be externally provided.
.circle.1 In the system arrangement, required are the timer circuit and the circuit for distributing three-system timer outputs to six switching elements of a power converter of a three-phase A.C. system, and these circuits externally attached may increase the production cost of the system and the possibility of the malfunction thereof due to noise mixing.
.circle.2 Since the timer and distribution circuits are provided externally from the microcomputer, the self-diagnosis function of the microcomputer, e.g., watch dog timer doesn't permit their abnormality to be detected. Therefore, another abnormality detection circuit is required for the circuits externally provided.
On the other hand, the latter system involves the problem that because of the increased amount of processing in the microcomputer, the minimum pulse width of the control signals to be supplied to the switching elements of the power converter cannot be reduced less than the interruption processing time in the microcomputer.